H3C Technologies H3C WX5500E Series Access Controllers - Manual (Page 179)

169
Device Comparison process
BPDU of port after
comparison
After comparison:
• Because the root path cost of CP2 (9) (root path cost of the
BPDU (5) plus path cost corresponding to CP2 (4)) is smaller
than the root path cost of CP1 (10) (root path cost of the
BPDU (0) + path cost corresponding to CP2 (10)), the BPDU
of CP2 is elected as the optimum BPDU, and CP2 is elected
as the root port, the messages of which will not be changed.
• After comparison between the configuration BPDU of CP1
and the calculated designated port configuration BPDU,
port CP1 is blocked, with the configuration BPDU of the port
unchanged, and the port will not receive data from Device A
until a spanning tree calculation process is triggered by a
new event, for example, the link from Device B to Device C
going down.
Blocked port CP2:
{0, 0, 0, AP2}
Root port CP2:
{0, 5, 1, BP2}
After the comparison processes described in Table 59 , a spanning tree with Device A as the root
bridge is established as shown in Figure 165 .
Figure 165 Final calculated spanning tree
BPDU forwarding mechanism in STP
The BPDUs are forwarded following these guidelines:
• Upon network initiation, every device regards itself as the root bridge, generates configuration
BPDUs with itself as the root, and sends the configuration BPDUs at a regular hello interval.
• If it is the root port that received a configuration BPDU and the received configuration BPDU is
superior to the configuration BPDU of the port, the device increases the message age carried in the
configuration BPDU following a certain rule, and it starts a timer to time the configuration BPDU
while sending this configuration BPDU out of the designated port.
• If the configuration BPDU received on a designated port has a lower priority than the configuration
BPDU of the local port, the port immediately sends out its own configuration BPDU in response.
• If a path becomes faulty, the root port on this path will no longer receive new configuration BPDUs
and the old configuration BPDUs will be discarded due to timeout. The device will generate
configuration BPDUs with itself as the root. This triggers a new spanning tree calculation process to
establish a new path to restore the network connectivity.